Explorations into the Effect of meso-Substituents in Tricarbocyanine Dyes: A Path to Diverse Biomolecular Probes and Materials

Abstract

Polymethine cyanine dyes have been widely recognized as promising chemical tools for a range of life science and biomedical applications, such as fluorescent staining of DNA and proteins in gel electrophoresis, fluorescence guided surgery, or as ratiometric probes for probing biochemical pathways. The photophysical properties of such dyes can be tuned through the synthetic modification of the conjugated backbone, for example, by altering aromatic cores or by varying the length of the conjugated polymethine chain. Alternative routes to shaping the absorption, emission, and photostability of dyes of this family are centered around the chemical modifications on the polymethine chain. This Minireview aims to discuss strategies for the introduction of substituents in the meso-position, their effect on the photophysical properties of these dyes and some structure-activity correlations which could help overcome common limitations in the state of the art in the synthesis.

Keywords: chemical probes; cyanine; fluorescence; imaging agent; meso-substitution.

Scheme 1

General synthesis of chain‐substituted cyanine dyes via a) condensation or b) ring‐opening of Zincke salts (above), where X=Cl, Br, I; R=alkyl, benzyl; R₁, R₂, R₃=various substituents, see for example ref. [10].

Figure 1

A comparison of selected crystal structures of different substituted cyanine dyes. Torsion angles refer exclusively to the meso‐position. Values of angles are in (°), distances are given in Ångström. (Note: due to the simplicity of this fragment, not all CCDC Conquest hits found in the CSD v. 5.41 database, updated 2020 were cyanine dyes.) Therefore, the classic definition was used, according to which cyanine dyes are nitrogen centers connected by conjugated double bonds. Merocyanines were excluded from the CCDC search. After disregarding the compounds which did not fulfil this criterion, polymorphs and different salts of the same basic cyanine dye, a total of 15 unique molecules were selected, most of which featured a simple meso‐Cl substituent.)

Scheme 2

Overview of common reactions to access structurally diverse cyanine imaging probes for biochemical species or processes and agents for photodynamic therapy.

Scheme 3

Synthesis of 2 and subsequent formation of the cytotoxic platinum complex 2‐Pt. Bottom left corner: photodegradation of 2‐Pt‐Platin, at 30′′ and 1′ intervals. Image used with permission from Ref. [23].

Scheme 4

Meso‐spiro reactivity of a tricarbocyanine dye. The authors described the use of 3 as an imaging agent for acidic microenvironments, often associated with inflammation. From left to right: control, LPS (lipopolysaccharides, provoke acute inflammatory response) with 3, and 3. Images used with permission from Ref. [26].

Scheme 5

Synthesis of meso‐amine tricarbocyanine dyes by Peng et al. Photophysical properties measured in water at a concentration of 1 μm. The fluorescence quantum yields were determined in reference to IR‐125 (for 4) and rhodamine B (for 5 and 6). ^[18c]

Scheme 6

Resonance structures of aminocyanine dyes (top), and proposed structure after treatment with base. Absorption and emission values according to Pascal et al. ^[8b]

Scheme 7

Synthesis of meso‐amine tricarbocyanine dye and subsequent amide formation using an acid chloride. Photophysical properties next to each dye. ^[31a] Counterions omitted for clarity.

Scheme 8

Top row: use of 10 as a switch‐on probe for nitric oxide in vivo. Counterions omitted for clarity. Bottom row: fluorescence imaging of living mice after subcutaneous injection of A) saline, B) NaClO₃, C) H₂O₂, D) DEA‐NONOate sodium salt (a nitric oxide donor), and E) LPS (as a model for inflammation). Three hours after injection, 10 was injected subcutaneously. Significant increase of signal intensity can be seen with DEA‐NONOate and LPS. Image used with permission from Ref. [37].

Scheme 9

Synthesis of N‐triazole‐functionalized cyanine dyes and comparison with a secondary amine substituent. Image used with permission from Ref. [18a]. Counterions omitted for clarity.

Scheme 10

Synthesis of oxygenated derivatives of rigid tricarbocyanine dyes and pH‐induced keto–enol equilibrium. Counterions were omitted for clarity.

Scheme 11

Derivatization of oxocyanine dyes by reaction with phosphoryl chloride and subsequent cleavage of the phosphorous−oxygen bond by alkaline phosphatase (ALP). Image adapted from Ref. [55] with permission. Counterions omitted for clarity.

Scheme 12

Derivatization of oxocyanines dyes by reaction with acid chlorides. Upon enzymatic reduction of the p‐nitro substituent of 21 by NTR, fluorescence intensity increased more than 100‐fold (22). ^[57] Counterions omitted for clarity.

Scheme 13

Reaction of meso‐Cl dye 9 with thio‐nucleophiles gives meso‐thioethers with both aliphatic (23) and aromatic substituents (24). ^[13b] Counterions omitted for clarity.